1. Field of the Invention
The field of the invention relates generally to ophthalmic lens products and more specifically to lenses and lens blanks for eyewear, the lenses preferably having a gradient tint that varies across the lens.
2. Description of the Related Art
Ophthalmic lenses are commonly used to correct vision errors, aberrations and focusing deficiencies caused by age, disease or other factors. In addition to correcting physiological vision problems, ophthalmic lenses and eyewear may also be used to ameliorate physical or environmental conditions (such as glare, variable lighting, high intensity light, dust, condensation, etc.) that can affect sight. Eyewear may also incorporate aesthetic features for fashion and style.
Gradient tints may be added to ophthalmic lenses for both fashion and function. Tint and polarization are often used to attenuate light exposure. At minimum, such attenuation can be soothing to eyes subjected to too much light intensity. In addition, selective filtering via tint or polarization may aid in sharpening contrast and allow better discrimination of objects or features. Linearly polarized lenses offer the additional and unique advantage of specifically blocking blinding glare caused by directional reflection and illumination.
To tint lenses, they are often dipped or submerged in dye tanks. Gradient tinting of eyewear lenses requires more precise, reproducible processing than solid tinting. This is due to the necessity to color match the left and right lenses in an eyeglass frame and to make sure that the positioning of the color or intensity variation is consistent on both lenses.
Unfortunately, many thermoplastic polymers, and particularly the popular polycarbonate lens material, absorb dyes from solution very poorly. Many of the common dyes are dissolved in water for ease of use, but polycarbonate tends to be hydrophobic. Many common organic solvents dissolve, discolor or structurally weaken polycarbonate and therefore may not offer viable alternative routes to introduce dyes. Additives such as surfactants, emulsifiers or various less-active organic solvents may improve dye absorption, but further improvements are still desired.
While additives may allow sufficient absorption of the dye into the polycarbonate to provide the desired color or attenuation initially, the dyes may be unstable to continued light exposure and may fade or migrate with time. The dyes may also be dissolved or removed during the subsequent hard coating process that is applied to polycarbonate lenses. Over-saturation of material tinting may allow sufficient color to remain in the final lens, but solution of the excess dye can unacceptably contaminate the coating process. Thus, alternative approaches to dye tanks have been sought.
Adding dye to the stock resin for polycarbonate injection-molding does not allow the manufacturing freedom to create a variety of differently colored products. It can be expensive and time-consuming to purge colored material from the injection molding machinery prior to running other colors or clear material. In addition, and importantly, many dyes cannot withstand the high temperature and pressure of polycarbonate injection molding. Other dyes will not dissolve or disperse evenly within polycarbonate and therefore this approach is of limited application. In addition, if one wants a gradient tint across the surface of the molded lens, it cannot be easily accomplished with dyed resin.
An alternative approach to achieve a solid tinted thermoplastic lens is to place a pre-tinted or polarized wafer or insert in the injection-molding cavity, and join it to the molten polycarbonate during the injection molding process. One significant problem with this approach is that the heat of the injection molding process may severely degrade the dyes. Burning or severe fading of the dyes in the gate areas where molten polycarbonate is first introduced is especially problematic. This problem is more severe with polycarbonate than with many other thermoplastic materials, due to the higher temperatures required for polycarbonate molding.
Another common problem is poor adhesion between the wafer or insert and the polycarbonate used for injection due to physical or chemical differences between these materials. This may occur even when the wafer or insert is made of polycarbonate because of the different processing the wafer or insert has undergone to form its polycarbonate sheet(s), in contrast to the pelletized material that is melted for injection molding. For example, it is not uncommon for the wafer or insert to be subjected to stretching, forming, dyeing, annealing or other processes prior to insertion into the mold cavity. It may also have a different molecular weight distribution, viscosity or different additives than the polycarbonate supplied for injection molding. For multilayer wafers, the necessary steps of lamination and/or treatment(s) for improved adhesion of these multilayer structures may create additional stresses within the wafer that reduce compatibility with the injected material.
U.S. Pat. No. 5,227,222 describes constructing a multilayer wafer with the improvement of placing an additional ink dissolution prevention layer on the inner surface to keep dye from dissolving into the thermoplastic molding material. However, extra layers increase the overall complexity and thickness of the final product, which may not be acceptable for eyeglass lenses. This additional complexity and cost is likewise pointed out in U.S. Pat. No. 6,117,384, which recommends using a single layer substrate due to the impracticality of multiple layers.
U.S. Pat. No. 8,012,386 describes a method of creating a gradient-tinted thermoplastic lens by a different approach involving the formation, from colored material, of a layer that decreases in thickness from its top to bottom edge, thus creating a varying tint down the part. This layer of varying thickness is then combined with another layer (with either no tint or a different tint) having complementary varying thickness (i.e., thicker at the bottom than the top) to give a constant total thickness over the whole lens. Such a configuration is not practical for prescription lenses because prescription lenses require thickness differences across the lens to create the vision correction lensing effect. In addition, this method requires multiple injection-molding steps to make the first colored layer with a wedge-like thickness, then change molding materials, and then form the second layer with a reversed wedge-like structure. Less involved manufacturing methods are desirable.
Thus, while several approaches have been suggested for tinting lenses or creating gradient tints, further improvements for such properties as robustness, ease of manufacture and practical application to a variety of ophthalmic lens configurations is desired.